1. Field of the Invention
This invention relates in general to an amplifier, and more particularly to a variable-gain wide-dynamic-range amplifier.
2. Description of the Related Art
In the field of communication, a receiver, for receiving signal, ordinarily adopts variable gain amplifier to amplify the signal being received. When the magnitude of the received input signal varies, the gain of an amplifier will be regulated accordingly to maintain proper magnitude of the output signal.
FIG. 1 is a circuit diagram showing a conventional variable gain amplifier using a current steering circuit. The variable gain amplifier 100 uses the current steering circuit 140 to regulate the gain of the amplifier 100 according to the voltage of the control signal VCTL. When the magnitude of the input signal Vi increases, the voltage of the control signal VCTL is decreased to reduce the magnitude of the output current Io so as to reduce the gain of the amplifier 100. However, the transistors Q121 and Q122 of the differential amplifier 120 will be saturated as the magnitude of input signal Vi becomes too large, which cause the output signal distorted. In this situation, problem involved with the distorted waveform of output current Io still remains even with the manipulation of decreasing the magnitude of the output current Io. It then comes to the acknowledgement of that the conventional variable gain amplifier 100 suffers the problem that the linearity is insufficient for meeting the generally requested needs. The mentioned linearity can be referred to as the range in which the magnitude of the input signal renders the output signal undistorted.
In order to increase the linearity, a conventional variable gain amplifier utilizing a degenerating resistance tuning circuit has been disclosed, as shown in FIG. 2. A metal oxide semiconductor field-effect transistor (MOSFET) serves as the degenerating resistance. The variable gain amplifier 200 controls the MOSFET 240 by adjusting the control voltage VCTL′ to regulate the equivalent resistance of the amplifier 200, so as to regulate the gain of the amplifier 200. Therefore, the range in which the magnitude of input signal Vi can be amplified without distortion by the amplifier 200 can be extended. In this manner, the dynamic range of the variable gain amplifier 200 becomes larger and the linearity of the variable gain amplifier 200 is increased. It is noted that the gain of the variable gain amplifier 200 is associated with the resistance of the MOSFET 240. However, due to the fact that the higher the frequency, the greater the bad influence caused by the parasite capacities existing in the degenerating circuit, i.e. the MOSFET 240. Therefore, the gain of the variable gain amplifier 200 in high frequency can not be precisely regulated. Moreover, it is difficult to control the linearity of the variable gain amplifier 200.
Besides, the U.S. Pat. No. 3,728,633 has disclosed a radio receiver with wide dynamic range. The receiver system of the patent includes two receivers. The first receiver is controlled by an automatic gain control (AGC) bias and the second by a delayed AGC bias. When the signal input level increases, the level of the AGC bias is raised. When the AGC bias increases and reaches a cross-over point, the gain of the first receiver is reduced to essentially zero and the second receiver is begun to operate. In this receiver system, the two receivers are switched by employing the AGC bias and the delayed AGC bias, instead of employing switches physically.
The U.S. Pat. No. 5,625,321 has disclosed a variable gain amplifier apparatus. This gain amplifier apparatus uses two variable gain amplifiers having complementary relation in their gain ratio. The two variable gain amplifiers are controlled by the same gain control signal. When the gain of one variable gain amplifier is increased, the other decreased.
To solve the above-mention problem, that is, the difficulty in controlling the linearity of variable gain amplifier, the U.S. Pat. No. 6,049,251 also has disclosed a very-wide-range variable-gain amplifier as shown in FIG. 3A. This very-wide-range variable-gain amplifier includes two amplifier circuits 320 and 340. FIGS. 3B and 3C are circuit diagrams showing the amplifier circuits 320 and 340, respectively. The resistance of resistor RE1 in FIG. 3B is smaller than that of resistor RE2, so that the amplifier circuit 320 has higher gain than the amplifier circuit 340 and lower linearity than the amplifier circuit 340. The amplifier circuits 320 and 340 are coupled in parallel, and controlled by auto gain control signals AGC1 and AGC2, respectively. Due to the different gains of the amplifier circuits 320 and 340, it is possible to obtain two ranges of the magnitude of input signal Vi which can be amplified without distortion and thus achieve an amplifier with high linearity.
However, if the range for the allowable input signal is to be further extended, it is necessary to have at least one additional amplifier circuit coupled to each other in parallel. In this manner, it will increase the number of elements, the cost and the complexity of the whole circuit. Thus, it is an important subject to extend the range for the allowable magnitude of the input signal, while the circuit complexity thereof is not increased.